Background Previous findings on the impact of co-occurring posttraumatic stress disorder (PTSD) in patients with borderline personality disorder (BPD) have revealed inconsistencies, which may have been related to small sample sizes or differences in the presence of childhood sexual abuse (CSA). In this study, the potentially aggravating impact of PTSD and the role of CSA were examined in a large cohort of BPD patients. Methods BPD patients with current PTSD (n = 142) were compared to BPD patients without PTSD (n = 225) regarding different BPD features such as non-suicidal self-injury. Further, we examined the potentially confounding role of CSA. Results BPD patients with PTSD showed elevated affect dysregulation, intrusions, dissociation, history of suicide attempts and self-mutilation compared to those with only BPD. The effects of PTSD on BPD patients regarding dissociation and the history of suicide attempts were at least partially related to CSA. Conclusions The additional diagnosis of PTSD in BPD patients can aggravate some, but not all BPD features. With respect to dissociation and suicide attempts, at least some of the impact seems to relate to CSA.